There are times when user equipment, such as a cellular phone, is communicating with a number of cells in a cellular phone system. One such time is during soft handover, which is a process of transitioning from one cell to another. However, there are other times when a user equipment will communicate with a number of cells. The set of all cells that “listen” to the user equipment during these times is called the active set. The active set can change over time, such that new cells are added to the set and old cells removed from the set.
During this communication process, the user equipment will transmit signals on one or more uplink channels and receive signals on one or more downlink channels with the cells in the active set. There are a number of messages, such as Automatic Repeat reQuest (ARQ) messages and messages concerning power received by the cell, that cells will communicate to the user equipment using signals on downlink channels. These messages include indicators having a number of predetermined states. For instance, the ARQ message has an indicator having an ACK (acknowledge) state and a NACK (no acknowledge) state. The user equipment receives these messages with no knowledge of what messages were originally sent, and the user equipment must assign with a high probability one of the predetermined states to the indicator of the received message.
In some systems, this decision is easily made. For instance, systems exist where an indicator is transmitted using values of −1 or +1, corresponding to two states. When these values can be detected with the same reliability, if a received message has a value below zero, a first state is chosen for the indicator and if a received message has a value above zero, a second state is chosen for the indicator. In this case, zero is being used as a dividing line. Although there is a probability that a+1 will be sent and the user equipment will incorrectly assign a−1 to the indicator (and vice versa), this probability is low and decreases with increasing power of transmitting the message and its associated indicator. In other systems, pilot symbols are used such that the downlink channel has a known power, which helps in the decision process. In still other systems, the downlink channel always has a non-zero power, such that zero does not have to be detected. Zero is the absence of transmission on the downlink channel, and is called DTX (discontinuous transmission).
A problem with some systems is that downlink channels can send −1, zero (i.e., DTX), or +1. If −1 or +1 is transmitted, the power at which the value is transmitted is unknown. The downlink channels can be transmitted to a single user equipment from many cells at the same time during a soft handover, and the powers from each cell can be different and unknown. Furthermore, the −1 and the +1 might have to be detected with a different reliability, which means that the zero (i.e., DTX) is of limited use as a dividing line.
It would therefore be desirable to provide techniques that remedy these problems.